Molybdenum (Mo) metal is usually found as deposits of molybdenite (MoS2) in nature. Traditionally, the metal value is extracted from the sulfidic ore through various pyrometallurgical techniques. The primary application for Mo is in the steel industry, where it is used to produce high strength steel alloys. The Mo used in these applications is supplied to the steel manufacturers either as molybdenum(VI) oxide (molybdenum trioxide, MoO3) or ferromolybdenum, an iron-molybdenum alloy.
In producing molybdenum trioxide, or so called technical grade oxide, the molybdenite is oxidized in various multiple hearth roasting furnaces. For example, one conversion process, referred to as the Climax conversion process, uses a Nichols Herreshoff or Lurgi design multiple hearth furnace to slowly roast the MoS2 at temperatures in the range of 530 to 700° C. (L. F. McHugh, P. L. Sallade, Molybdenum Conversion Practice, Metec, Inc.: Winslow N.J., 1977; L. F. McHugh, J. Godshalk, M. Kuzior, Climax Conversion Practice III, The Metallurgical Society of CIM, 1977, 21-24). Over the course of roasting, the molybdenite is first converted to molybdenum(IV) oxide (molybdenum dioxide, MoO2) and then slowly converted to the trioxide, MoO3, in the lower hearths. In such a roasting operation, the lower hearths are maintained at lower temperatures to keep the highly volatile MoO3 from sublimating. The SO2 produced during conversion, which is highly diluted due to the significant amount of excess air used during roasting, is usually converted to sulfuric acid in a downstream process, which may result in some high pressure steam production.
The molybdenite host ore (unprocessed ore containing molybdenite) is usually concentrated prior to oxidation to upgrade the molybdenite concentration via an oil flotation method. A method has been described wherein the residual flotation oil can be removed in situ during roasting to produce technical grade oxide from the molybdenite without further purification (L. F. McHugh, D. E. Barchers, Roasting of Molybdenite Concentrates Containing Flotation Oils, U.S. Pat. No. 4,523,948, issued Jun. 18, 1985).
In alternative incarnations of MoO3 production, microwaves have been used to heat molybdenite host ores in the presence of oxygen to convert them to oxides, followed by separation and recovery (P. R. Kruesi, V. H. Frahm, Jr., Process for the Recovery of Molybdenum and Rhenium from their Sulfide Ores, U.S. Pat. No. 4,321,089, issued Mar. 23, 1982). Chlorine can be substituted for oxygen and the metals recovered as chlorides. The conversion can be conducted in a flash roasting set-up wherein the molybdenite is converted to gaseous MoO3 and any slag-forming constituents are converted to a liquid slag that separates from the molybdenum, wherein the flash roasting is performed at 1600±200° C., and the technical grade oxide is later condensed at significantly lower temperatures (B. J. Sabacky, M. T. Hepworth, Flash Roasting of Molybdenum Sulfide Concentrates in a Slagging Reactor, U.S. Pat. No. 4,555,387, issued Nov. 26, 1985). High pressure oxidation carried out in an autoclave followed by leaching and recovery has also been described (R. W. Balliett, W. Kummer, J. E. Litz, L. F. McHugh, H. H. K. Nauta, P. B. Queneau, R.-C. Wu, Production of Pure Molybdenum Oxide from Low Grade Molybdenite Concentrates, U.S. Pat. No. 6,730,279, issued May 4, 2004).
It is more attractive to the steel industry to use MoO2 rather than MoO3 because the lower oxygen content means a higher Mo content, and less reducing agent consumption during the production of molybdenum steels. Additionally, the dioxide is less volatile than the trioxide. The use of MoO2 also eliminates the need to produce ferromolybdenum. Several authors have attempted to produce the dioxide from molybdenite concentrates using various techniques.
For example, by mixing stoichiometric amounts of powdered or pelletized MoO3 and MoS2, MoO2 can be produced at temperatures of 600 to 700° C. and pressure slightly in excess of atmospheric pressure, while liberating SO2 (R. Cloppet, Process for the Production of Molybdenum Dioxide, U.S. Pat. No. 3,336,100, issued Aug. 15, 1967). The molybdenum product was then further treated in an SO2 lean atmosphere to produce the final dioxide product. In a similar process, particulate MoO2 and MoS2 (weight ratio 2:1) were roasted at 700 to 800° C. in the presence of enough oxygen to facilitate the conversion of the molybdenite to MoO2 (B. J. Sabacky, M. T. Hepworth, Molybdenum Dioxide-Molybdenite Roasting, U.S. Pat. No. 4,462,822, issued Jul. 31, 1984). A portion of the dioxide produced was recycled to convert the next charge of molybdenite. Some MoO3 is produced as a by-product. Pelletized MoO2 has been produced from MoO3 using a reducing H2 atmosphere in a reaction vessel that was able to control the exothermic reduction of the trioxide (H. W. Meyer, J. D. Baker, W. H. Ceckler, Direct Reduction of Molybdenum Oxide to Substantially Metallic Molybdenum, U.S. Pat. No. 4,045,215, issued Aug. 30, 1977). The dioxide was further reduced to metallic Mo, or the dioxide was collected as a final product.
It has also been proposed to convert molybdenite to MoO2 using water steam (K. Y. Hakobyan, H. Y. Sohn, A. V. Tarasov, P. A. Kovgan, A. K. Hakobyan, V. A. Briovkvine, V. G. Leontiev, and O. I. Tsybine, “New Technology for the Treatment of Molybdenum Sulfide Concentrates,” Sohn International Symposium Advanced Processing of Metals and Materials Vol. 4 to New, Improved and Existing Technologies: Non-Ferrous Materials Extraction and Processing, ed. by F. Kongoli and R. G. Reddy, TMS (The Minerals, Metals & Materials Society), pp. 203-216, 2006; H. Y. Sohn, Process for Treating Sulfide-Bearing Ores, U.S. Pat. No. 4,376,647, issued Mar. 15, 1983; K. Y. Hakobyan, P. A. Kovgan, A. V. Tarasov, A. K. Hakobyan, Eurasian Patent 002417, issued 2002; K. Y. Hakobyan, H. Y. Sohn, A. K. Hakobyan, V. A. Bryukvin, V. G. Leontiev, and O. I. Tsibin, “The Oxidation of Molybdenum Sulfide Concentrate with Water Vapor: Part I. Thermodynamic Aspects,” Mineral Processing and Extractive Metallurgy (TIMM C), 116, 152-154 (2007); K. Y. Hakobyan, H. Y. Sohn, A. K. Hakobyan, V. A. Bryukvin, V. G. Leontiev, and O. I. Tsibin, “The Oxidation of Molybdenum Sulfide Concentrate with Water Vapor: Part II. Macrokinetics and Mechanism,” Mineral Processing and Extractive Metallurgy (TIMM C), 116, 155-158 (2007)). In such a scheme, steam is reacted with the MoS2 feed to produce the dioxide and an H25 off-gas stream. Hydrogen sulfide is a toxic gas and is difficult to handle in downstream processes. A rotating furnace is used to treat molybdenite ores at 900 to 1,000° C. in a countercurrent flow of water vapor in an excess of 6 to 10 times the mass of the molybdenite. Residual sulfur is optimized to minimize the loss of MoO2 to MoO3 vapor formation.
Solid MoO3 has been used in a reaction between MoO3 and MoS2 to produce MoO2 (L. F. McHugh, D. K. Huggins, M. T. Hepworth, J. M. Laferty, Process for the Production of Molybdenum Dioxide, U.S. Pat. No. 4,552,749, issued Nov. 12, 1985). In this process the conversion to dioxide is carried out at temperatures low enough to favor dioxide formation and to produce an SO2-rich stream (750 to 950° C.). A portion of the MoO2 product was sent to a flash reactor where it was reoxidized to the trioxide at temperatures sufficient to sublimate the MoO3 (1,000 to 1,700° C.) and recycled to convert the next charge of molybdenite. In a similar process, the MoO2 is oxidized in a second step to MoO3 to recover rhenium from the final product; in this operation the MoO2 is not the final product (H. Y. Sohn, Process for Treating Sulfide-Bearing Ores, U.S. Pat. No. 4,376,647, issued Mar. 15, 1983).
Solid MoO3 has been proposed as an oxidant to molybdenite to achieve a high degree of desulfurization; the process required ca. 10% excess MoO3 to be mixed intimately with the molybdenite in order to achieve a low sulfur content product (L. F. McHugh, R. Balliett, J. A. Mozolic, The Sulfide Ore Looping Oxidation Process: An Alternative to Current Roasting and Smelting Practice, JOM, July 2008, 84-87; L. F. McHugh, Oxidation of metallic materials as part of an extraction, purification, and/or refining process, U.S. Patent Application 2008/0260612 A1, published Oct. 23, 2008). Some of the dioxide was then reoxidized in a flash furnace with oxygen to recycle MoO3 to the molybdenite charge (similar to as reported in L. F. McHugh, D. K. Huggins, M. T. Hepworth, J. M. Laferty, Process for the Production of Molybdenum Dioxide, U.S. Pat. No. 4,552,749, issued Nov. 12, 1985).
The methods described above all appear to use significant excess oxygen. While these methods produce MoO2 from molybdenite feeds, it has not been possible to produce a product without concomitantly producing some MoO3. When it has been used, oxygen has also always been in excess. A method that can produce exclusively MoO2 is desired.
There accordingly remains a need in the art for methods for the production of MoO2 from molybdenite. Producing MoO2 with lower amounts of MoO3 is considered particularly advantageous. It is further desired that the MoO2 productions methods allowed control of SO2 and energy consumption during production.